1. Field of the Invention
The present invention relates generally to methods and apparatus for suspending an object having weight relative to a base, and for isolating or reducing the transmission of vibrations between the object and the base on which the apparatus is supported, and more particularly, to an omni-directional suspension or vibration-isolating system, which exhibits low stiffness both vertically in the direction of the weight load and horizontally in directions transverse to the weight load, to effectively reduce the transmission of vibrations between the object and the base.
The invention also relates to other applications such as simulating a zero-gravity environment or for optical pointing or measurement systems or any system where it is desirable to provide small motions of a suspended object without resistance from spring forces or mechanical friction.
2. Description of Related Art
Much work and development have been directed to the problem of effectively isolating or reducing the effects of unwanted environmental vibrations on motion-sensitive devices such as machines and instruments. When transmitted to motion-sensitive devices through the foundation or structure of a building, or through the ground itself, environmental vibrations can adversely affect the testing, calibration, and performance of these devices. Road and rail traffic, for example, provide an ample source of unwanted vibrations. Also, micro-seismic ground motions are always present, even in remote areas.
A building itself can also generate and transmit unwanted vibration to the sensitive equipment located within it. For example, seismic activity or wind conditions around the building can cause it to vibrate. Since such motion can be further amplified on the upper floors of a multi-story building, sensitive equipment is often placed on lower floors or on slabs or seismic masses isolated from the rest of the building and resting on or isolated from the soil itself. Additionally, unwanted vibrations can be generated throughout the building by its heating, ventilating and air conditioning system. Foot traffic of workers and other machinery are still further examples of sources of vibrations that can adversely affect the performance of motion-sensitive equipment.
Vibrations can also cause problems outside of the setting of the conventional workplace. For example, noisy equipment on submarines can be transmitted to the hull, causing it to vibrate and transmit acoustic or vibrational energy through the water, making the submarines more susceptible to detection.
Earthquakes can also cause buildings and other equipment to fail by exciting vibrations in the buildings or equipment at their natural frequencies coinciding with frequencies of the ground motions.
Numerous devices have been developed for reducing the transmission of unwanted vibrations between an object and its surroundings. These devices, commonly called vibration isolators, are designed as a support structure that can be placed between the object to be isolated and the foundation. Isolators exhibit stiffnesses or spring rates such that objects supported on them have natural frequencies of vibration substantially lower than the frequencies of the unwanted vibrations. Generally, the higher the frequencies of the unwanted vibrations relative to the natural frequencies of the object supported on the isolators, the more effective the isolators in isolating these vibrations.
The measure of the ability of an isolator to either amplify or suppress an input vibration is called the transmissibility of the isolator. Transmissibility is defined, for a vibrating foundation, by the ratio of the response amplitude of the isolated object to the excitation amplitude of the foundation, or, for a vibrating object, by the ratio of the amplitude of the force transmitted to the foundation to the amplitude of the exciting force on the object.
FIG. 25 shows curves of transmissibility vs. frequency ratio for an ideal single-degree-of-freedom isolation system consisting of a rigid payload supported on an isolator consisting of a linear spring and a viscous damper. The various curves correspond to various amounts of damping. The frequency ratio is the ratio of excitation frequency to natural frequency. These curves show that such an isolator amplifies the vibrations for frequency ratios below about 1.4 and isolates the vibrations for higher frequency ratios, with greater isolation for greater frequency ratios. That is, for a given frequency of vibration, lower natural frequencies of the system result in better isolation. Also, the curves show that for a given frequency of vibration and a given frequency of the system, lower damping results in better isolation, although damping is often required to limit the resonant amplification.
Transmissibility curves for real isolators follow the curves of FIG. 25 quite well until excitation frequencies increase to the point where isolator resonances or surge frequencies occur. That is, at sufficiently high frequencies the isolators resonate or vibrate at their own natural frequencies, so transmissibility curves of real isolators look like that of FIG. 26 which is typical of a mechanical spring. Isolator resonances occur because isolators have mass as well as stiffness and, in general, for a given isolator stiffness a greater isolator mass will result in lower isolator resonances. It is apparent from FIGS. 25 and 26 that, for a given frequency of excitation, isolation is improved by reducing the isolator stiffness, and therefore the system natural frequency, as much as possible while keeping the isolator resonances above the frequency of excitation. In general this results in conflicting requirements for isolator designs in the prior art. With linear springs, for example, as the spring stiffness is reduced, the deflection under the payload weight increases, with an accompanying increase in stored elastic energy. Since springs can only store a limited amount of energy per unit mass of spring and remain elastic, increased energy storage requirements for the spring increases its required mass and reduces its resonant frequencies. Therefore, prior art isolators generally have a limited separation between the lowest isolation system natural frequencies attainable and the highest isolator resonant frequencies attainable.
Unwanted vibrations occur in a wide range of frequencies and amplitudes and there is increasing use of extremely motion-sensitive instruments and equipment that needs to be isolated from these vibrations in frequency ranges and amplitudes where prior art isolators are not effective. Typical applications include microelectronics manufacturing, machining to extreme tolerances and industrial laser and optical systems.
High-frequency vibrations produced by machinery, for example, can be effectively isolated by a variety of prior art isolation system techniques such as resilient pads made from of a variety of materials and various types of mechanical springs. When vibration input frequencies are very low, such as 5 Hz or lower, the choices of prior art solutions become much more limited, particularly when the vibration amplitudes are very small, such as a micro-g of acceleration or a micron of displacement.
Air springs are typically used where the lowest frequencies and highest isolation performance are required, but they have limitations. The lowest natural frequencies of typical high-performance air springs are limited to about 1.0-1.5 Hz and even higher, particularly with micro-motion inputs. Also, high-performance air springs have other problems, namely, they leak, they require air or power supplies and leveling valves which can be a maintenance nuisance and they can contaminate clean rooms and generally cannot be used in hard vacuums.
Accordingly, those concerned with the development and use of vibration isolation systems and apparatus have long recognized the need for improved vibration isolation, particularly in the low frequency ranges. Preferably, an improved device should be compact and relatively light in weight and should provide improved vibration isolation at low and high frequencies in both vertical and horizontal directions. Also, such a system or apparatus should be able to isolate vibrations of extremely small amplitude, such as micro-seismic motions, without significant loss of performance.